The following abbreviations are herewith defined. At least some of which are referred to within the following description.
3GPP: Third Generation Partnership Project
BLER: Block Error Rate
CE: Coverage Enhancement
CRC: Cyclic Redundancy Check
C-RNTI: Cell Radio Network Temporary Identifier
DCI: Downlink Control Information
DL: Downlink
ECCE: Enhanced Control Channel Element
EREG: Enhanced Resource Element Group
eMTC: Enhanced Machine-Type Communications
eNB: Evolved Node B
EPDCCH: Enhanced Physical Downlink Control Channel
FDD: Frequency-Division Duplex
LTE: Long Term Evolution
M-PDCCH: Machine-Type Communications Physical Downlink Control Channel
MTC: Machine-Type Communications
OFDM: Orthogonal Frequency Division Multiplexing
PCFICH: Physical Control Format Indicator Channel
PCID: Physical Cell Identity
PDCCH: Physical Downlink Control Channel
PDSCH: Physical Downlink Shared Channel
PHICH: Physical Hybrid-ARQ Indicator Channel
PRACH: Physical Random Access Channel
PRB: Physical Resource Block
PUCCH: Physical Uplink Control Channel
PUSCH: Physical Uplink Shared Channel
RA-RNTI: Random Access Radio Network Temporary Identifier
RACH: Random Access Channel
RAR: Random Access Response
RNTI: Radio Network Temporary Identifier
RSRP: Reference Signal Received Power
RRC: Radio Resource Control
SC-FDMA: Single Carrier Frequency-Division Multiple Access
SIB: System Information Block
TB: Transport Block
TC-RNTI: Temporary Cell Radio Network Temporary Identifier
TDD: Time-Division Duplex
TMSI: Temporary Mobile Subscriber Identity
UE: User Entity/Equipment (Mobile Terminal)
UL: Uplink
WiMAX: Worldwide Interoperability for Microwave Access
Machine-type communications (MTC) via wireless communication networks, such as LTE systems, enable data services for wearable electronic devices, smart appliances, smart meters, and so forth. Enhanced MTC (eMTC) allow for 15-20 dB more enhanced coverage compared to existing LTE networks and lower power consumption. To support the 15-20 dB more enhanced coverage, repetition of control channels and data channels are used along with soft combining in the receiver. The bandwidth of an eMTC user equipment (UE) is reduced to 1.4 MHz in both downlink and uplink. In certain configurations, a single eMTC UE can tune to different 1.4 MHz narrow bands across the whole system bandwidth over time. As may be appreciated, the eMTC UEs cannot receive PDCCH (because PDCCH spans across the whole system bandwidth as defined by 3GPP) in a system with a bandwidth larger than 1.4 MHz. Using M-PDCCH (which is based on EPDCCH as defined by 3GPP) instead may be a natural solution for transmitting a control message to an eMTC UE. M-PDCCH may only be transmitted on a subset of the PRBs within the system bandwidth.
As the number of eMTC UEs is expected to be high, access congestion may occur, particularly when the eMTC UEs perform random access around the same time. For example, with high numbers of eMTC UEs initiating random access around the same time, congestion of the contention resolution message, also referred to as Msg4, may become severe.
An eMTC UE initiates random-access by transmitting a preamble sequence to an eNB on the random access channel (RACH) (also referred to as Msg1) in a time-frequency resource. The PRACH time-frequency resource is configured by higher layers, e.g. broadcasted in a system information block (SIB). The eMTC UE selects one of sixty-four (64) RACH preambles to transmit in a PRACH time-frequency resource. Consequently, it is possible for two or more eMTC UEs to send the same preamble sequence in the same frame. In response to receiving a Msg1, the eNB sends a random access response (RAR) (also referred to as Msg2). The RAR includes a temporary cell radio network temporary identifier (TC-RNTI) assigned to the eMTC UE and an uplink (UL) grant. The Msg2 is transmitted on the physical downlink shared channel (PDSCH), and is scheduled by a DL grant transmitted on an MTC physical downlink control channel (M-PDCCH). The M-PDCCH used to carry the DL grant scheduling the RAR is transmitted in a time-frequency resource determined according to a pre-determined rule. The time-frequency resource used by the PDSCH carrying the RAR is indicated by the DL grant scheduling the RAR.
After receiving the RAR, the eMTC UE sends a radio resource control (RRC) connection request (also referred to as Msg3) using the UL grant. The frequency resource for transmission of Msg3 is indicated by the UL grant. In response to receiving the Msg3, the eNB sends a contention resolution message (also referred to as Msg4). The Msg4 is transmitted on the PDSCH, and is scheduled by a DL grant transmitted on the M-PDCCH. After sending the Msg3, the eMTC UE initiates the timer (mac-ContentionResolutionTimer) and continuously monitors the M-PDCCH for a DL grant scheduling the Msg4 until either the Msg4 is received or the timer expires. Conventionally, a UE monitors the same frequency resource for receiving the DL grant scheduling the Msg2 and the DL grant scheduling the Msg4. For an eMTC UE with coverage enhancement, the transmission of its M-PDCCH and PDSCH may be repeated over a number of subframes. In case multiple eMTC UEs with coverage enhancement initiate random access procedure, the M-PDCCH capacity may become limited for transmission of the DL grants scheduling Msg4 for multiple eMTC UEs. Consequently, an eMTC UE may need to monitor the M-PDCCH for DL grant scheduling Msg4 in much more subframes, which increases its power consumption.